This invention relates in general to the field antennas, and in particular to thin film and printed circuit resistive loading of spiral, sinuous, or similar antennas.
Spiral and sinuous antennas are important in a number of areas, especially in direction finding, surveillance systems, and electronic countermeasure systems. In general, they are useful in low profile circular polarization applications, including communications.
Two arm planar, cavity backed spiral antenna structures with unidirectional rotationally symmetric patterns have proved to be particularly valuable. The cavity for such an antenna is typically filled with absorbing material to achieve wide bandwidths. Sinuous antennas denote antennas in the shape of curves, curves and sharp turns or bends, or straight lines and sharp turns, with the sharp turns or bends occurring in an alternating fashion (such as a "zig-zag" pattern).
Resistive termination of the arms of a spiral, sinuous, or similar antennas is necessary because any finite antenna suffers from arm-end reflections which degrade the low frequency impedance of the antenna. Resistive termination suppresses unwanted currents introduced in cavity-backed spiral, sinuous, or similar antennas.
Customary approaches for resistive termination of the arms of such antennas involve the use of resistive paint on each arm near the region of truncation, the use of lumped resistors on the end of each arm, or the use of volumetric absorbers near the end of each arm. All of these schemes require processing and/or parts additional to the printed circuit arms. In addition, volumetric or resistive paint schemes are relatively clumsy, imprecise, and often produce comparatively abrupt discontinuities in the radiation pattern of the antenna. Volumetric absorbers also require machining, installing, and bonding. Lumped resistors typically are limited to lower frequencies and are clumsy to implement. Lumped resistors also do not provide for changing value across the bandwidth of the antenna.